Quantitative Impact of Coil Misalignment and Misplacement in Transcranial Magnetic Stimulation

IntroductionTargeting in transcranial magnetic stimulation (TMS) involves the accurate placement and positioning of the stimulation coil on the head of a subject or patient. In clinical and research applications, this placement is even done manually and/or with fixed coil holders that do not compensate for motion and drift of the head. The placement involves six degrees of freedom (DOF; three position DOF: 1x contact and 2x head location; three rotational DOF: 2x alignment and 1x electric field direction/orientation), which challenge operators. This procedure is--even with an experienced user--prone to positioning errors, which can result in low treatment efficacy or high stimulation strength due to overestimating the resting motor threshold (RMT). Whereas the position and field orientation are at least widely appreciated, the coil-head alignment and its impact are often not even known. Errors involve constant errors, drift (both leading to bias and inter-individual variability), and particularly fluctuations (causing intra-individual variability). ObjectiveWe demonstrate the impact of positioning error on cortical field strength to get a better understanding of the importance of accurate positioning and compare as well as quantify the impact of position vs. alignment errors. MethodsWe simulated the impact in a realistic head anatomy to quantify various levels of position errors and misalignment, rolling-off the coil from the target. ResultsPosition and alignment errors shift the focus of the electric field and reduce the electric field in the actual target. A misalignment of 10{degrees} can exceed the loss of stimulation strength in the target associated with a shift of 10 mm, corresponding to threshold stimulation leading to no detectable electromyographic response anymore. Misalignment in the direction of the handle (pitch), with which many operators appear to struggle most, reduces the field in the actual target faster than left-right roll. ConclusionThis work highlights the importance of the coil-head alignment for intra- and interindividual variability.